I don?t have Vista so I am not familiar with the problem. But try this.
Open one of the Word documents on your Vista computer. Then click on File, then, Save As, and look to see what file format options are available to you. You will probably find an option to save in a pre Vista Word format which would be compatible with XP. But save using a different file name so as not to change the format of your original file.
The same principle should apply to your Excel files.